Afghanistan Agriculture UC Davis Managing Crop Disease

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Unit G: Pest Management
Lesson 2: Managing Crop Diseases

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Terms
Abiotic disease
Bacteria
Biotic disease
Cultural disease control
Disease avoidance
Disease resistance
Disease tolerance
Fungi
Infectious diseases
Localized infection
Nematodes
Noninfectious disease
Pathogen
Plant disease
Plant pathology
Systemic infection
Viruses

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What is a plant disease & what conditions
are necessary for it to develop?
A plant disease is a harmful alteration of the
normal physiological and biochemical growth
of the plant, or as a condition in which a plant
differs from a normal (healthy) plant in
appearance, structure, or function.
Plant pathology is the study of plant
diseases.

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Plant diseases differ
from nonparasitic injury
in the length of time
during which they affect
the plant.
A disease usually
consists of a series of
harmful processes that
occurs over a fairly long
period of time. Injuries
are disorders that occur
over a short period of
time.
Pest problems develop when
three conditions are
present:
• pest (insect, weed,
disease, etc.)
• host (susceptible plant)
• favorable environment

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Four conditions
necessary for a plant
disease to develop are:
a susceptible host
plant, a disease-
producing agent, a
favorable environment,
and a time for the
disease to develop.
If any one of these
conditions is not met, a
disease will probably
not occur.

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How are plant diseases
classified?
Plant diseases can be divided into two
broad categories
Plant diseases are classified as either
noninfectious or infectious, depending
on their cause.

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Noninfectious disease
or abiotic disease is
not caused by a
pathogen, but rather by
elements in a plant’s
environment that are
damaging to it, such as
weather, air pollutants,
and nutrient
deficiencies.
They cannot be
transmitted from plant
to plant but they occur
very quickly in a wide
variety of plants.
Infectious diseases or
biotic diseases are
caused by a living
organism or pathogen.
A pathogen is a living,
disease-producing
agent that can be
transmitted from plant
to plant.
They may invade the
entire plant (systemic
infection) or only affect
certain plant parts
(localized infection).

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Bacteria are
microscopic one-celled
organisms that
reproduce by simple
division.
One bacterium can
produce up to 70 billion
offspring in hours.
Bacteria cannot move
by themselves and
depend on splashing
rain, wind, animals,
insects, implements,
seed, and other means
to get to host plants.
Bacteria usually enter
through wounds in the
plant and cause local or
systemic infections.

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Bacteria

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Fungi are the most
common causes of
infectious plant
diseases.
Fungi are small, many-
celled plants that lack
chlorophyll.
Most reproduce by
spores. Fungi enter the
plant and destroy or
disrupt parts of it,
making it unsuitable for
human use.
The organisms are
spread by rain, wind,
insects, seeds, farm
implements, and runoff
water or runoff soil.
Symptoms of fungal
diseases included
wilting, yellowing,
blotching, spotting of
leaves, and rotting of
roots, stalks, stems, or
fruit.

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Fungi

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Nematodes are microscopic roundworms
that live in the soil and feed in the root system
of plants.
Nematodes are obligate parasites in that they
can survive only on or in living plants.
They reproduce by eggs and are spread by
anything that moves nematode-infected plant
parts or nematode-infested soil.

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Nematodes

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Viruses are tiny
pathogens that can be
seen only with special
microscopes.
Viruses usually enter
plants and cause
systemic infections.
Viruses cannot move on
their own, and are
commonly spread by
insects and seed.

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BACTERIA
wilt (bacterial) corn, alfalfa, tomato, potato
gall crown gall in trees and many crops
Infection angular leaf spot of cotton
FUNGI
Anthracnose cotton, cucumber, cantaloupe, watermelon
downy mildew grain crops, grape, onion, spinach, lettuce, cucumber
powdery mildew grain, cucumber
wilt cotton, tomato, sweet potato, watermelon
VIRUSES
Mosaic tomato, potato, sugarcane stuntcorn
Streak sugarcane

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How are plant diseases scouted,
identified, and diagnosed?
Plants react to pathogens by producing
symptoms, indications of disease that
affect the external or internal
appearance of the plant.

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As a field is spot-
checked for insects, the
severity of diseases can
also be noted.
1. Examine roots,
stalks, and leaves, and
collect samples for
positive identification. It
is important to inspect
the entire plant when
diagnosing a plant
disease problem.
2. Fields are normally
spot-checked in five
different areas in 20-
foot sections or within a
1 by 10 foot area for
forage & small grains.

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Determine the severity
of the disease and the
percentage of plants
displaying disease
symptoms.
1. Some common
symptoms of plant
pathogens are wilting,
yellowing, leaf spots,
blights, dropping
leaves, and necrosis or
death of plant tissue.
2. The pathogen itself
may also produce
signs.

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To identify plant
diseases correctly, you
must carefully observe
the symptoms of the
disease and the signs
of the pathogen itself.
The symptoms are
usually of three types.
1. Over-development of
tissues: galls and
swellings.
2. Underdevelopment of
tissues: stunting, lack of
chlorophyll, or
incomplete
development of organs.
3. Death of tissues: leaf
or flower blights, leaf
spots, root rots,
cankers, wilting.

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Examine all parts of the
injured or diseased
plant.
Root problems may
produce wilting,
stunting, dieback, or
nutrient deficiencies.
Determine whether the
problem is localized or
systemic.
Some pathogens infect
only certain parts of the
plant.
Diseases that affect the
entire plant are called
systemic infections.
Once inside the plant,
the pathogen moves
throughout & causes
wilting, yellowing, and
stunted growth.
Systemic diseases
often kill the plant.

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Steps to follow for diagnosing
diseases.
1. Scout the field and
note problem areas.
2. Examine the plants
and note the plant parts
affected, symptoms of
disease, & signs of
pathogens.
3. Observe the field and
note the infestation
pattern, field conditions,
field history, & weather
conditions for the past
10 to 14 days.
4. Consult references to
assist you in disease
identification or consult
your county extension
adviser.

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The methods used to control crop
diseases are many but can be grouped
into three main categories
1. Disease avoidance is when crop plants
may have morphological structures such as
sunken stomata or a thick cuticle that
discourages penetration of inoculums.
2. With disease tolerance an infection may
occur, but the crop host is able to withstand
the invasion, continue to grow, and produce
satisfactory yields. There is no evidence of
incubation and/or infection even though
inoculation has occurred with true disease
resistance.

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The methods used to control crop
diseases are many but can be grouped
into three main categories
3. Cultural disease control involves
any change or manipulation of field
operations that alters either the life
cycle of the pathogen or the host, so
that inoculation or infection does not
occur.

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Optimum plant density, timely cultivation
and fertilization, and proper water
management are all practices that
promote vigorous crop growth. Healthy
plants are less disposed to disease
while stressed plants are more likely to
suffer disease infection and damage.

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1. Crop rotations change the crop host and
also reduce the level of the primary inoculum.
2. A change in planting date, either earlier or
later, often helps a crop escape or withstand
disease infection.
3. Tillage and cultivation reduce the source of
primary inoculum of some diseases by
burying crop residues.
4. Weed control of all kinds often eliminates
the alternate disease host but always reduces
the competition of weeds for light, water, and
nutrients.

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5. Chemical disease control may be effective
but it is not always feasible on field crops.
Chemical control must be applied before
inoculation and remain on the plant or be
reapplied as long as there is a threat of
inoculation. Once a pathogen has entered the
plant, chemical control is difficult, if not
impossible. Chemical control of plant
diseases is strictly preventative. There are no
treatments to reduce the symptoms once a
plant has the disease.

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What strategies are used in
disease management?
Successful management of field crop
diseases is based on a thorough
understanding of factors influencing
disease development and expression.
Strategies should include measures to
reduce losses in the current crop as well
as considerations for future plantings.

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Review/ Summary
1. How do you identify the conditions
necessary for a plant disease to
develop?
2. What are some classification of plant
diseases?
3. How do you control plant diseases?
4. What strategies are used in disease
management?